RAISED ZONE RETROREFLECTIVE CUBE CORNER ARTICLE AND METHOD OF MANUFACTURE

Abstract: A cube corner article and method of manufacturing the article from a series of unitary substrates. The method of manufacture comprises the first step of providing an initial non-unitary cube corner element array comprising a plurality of geometric structures including cube corner elements (212, 218). Then, a replica of the cube corner element array is produced as a substrate suitable for forming retroreflective surfaces. Portions of the replica substrate material are removed. The removal of these portions forms at least one cavity (206) bounded by side walls in the replica at a depth at least that of the cube corner elements. The replica is replicated to produce an additional directly machinable substrate suitable for forming retroreflective surfaces. The substrate comprises at least one raised section (208) having sidewalls at a height at least that of the cube corner elements. At least one raised section is directly machined to form a zone comprising a plurality of geometric structures including cube corner elements bounded by at least two sets of parallel grooves.

Description: RAISED ZONE RETROREFLECTIVE CUBE CORNER ARTICLE AND ΈΓHOP OF MANUFACTURE Cross-Reference to , ited Application This is a Continuation in Part of U.S. Patent Application Serial No. 08/139,448, Raised Zone Retroreflective Cube Corner Article and Method of Manufacture, filed October 20, 1993. Field of the Invention This invention relates to retroreflective articles having prismatic retroreflective elements. Back round Many types of retroreflective articles are known, and are made in a variety of ways. One common type of retroreflective article uses transparent microspheres, typically with hemispheric retroreflectors thereon. Examples of this type of retroreflector are disclosed in U.S. Patent 2,407,680 (Palmquist), U.S. Patent 3,190,178 (McKenzie), and U.S. Patent 4,025,159 (McGrath). Another type of retroreflective article includes prismatic designs incorporating one or more structures commonly known as cube corners. Retroreflective sheeting which employs cube corner type reflective elements is well known. An example of such designs is shown in U.S. Patent 3,684,348 (Rowland). The manufacture of retroreflective cube corner element arrays is accomplished using molds made by different techniques, including those known as pin bundling and direct machining. Molds manufactured using pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner retroreflective element. For example, certain pin bundled arrays permit elaborate assembly into various pin structural configurations. U.S. Patent No. 3,926,402 (Heenan et al) and U.S. Patent No. 3,632,695 (Howell) are examples of pin bundling. The direct machining technique, also known generally as ruling, comprises cutting portions of a substrate to create a pattern of grooves which intersect to form cube corner elements. The grooved substrate is referred to as a master mold from which a series of impressions, i.e. replicas, may be formed. In some instances, the master is useful as a retroreflective article, however replicas, including multi- generational replicas, are more commonly used as the retroreflective article. Direct machining is an excellent method for manufacturing master molds for small micro-cube arrays. Small micro-cube arrays are particularly beneficial for producing thin replica arrays with improved flexibility, such as continuous rolled goods for sheeting purposes. Micro- cube arrays are also more conducive to continuous process manufacturing. The process of manufacturing large arrays is also relatively easier using direct machining methods rather than other techniques. One example of direct machining is shown in U.S. Patent No. 4,588,258 (Hoopman). Summary of Invention The invention comprises a method of manufacturing a cube corner article comprising the steps of providing an initial non-unitary cube corner element array comprising a plurality of geometric structures including cube corner elements, producing a replica of the cube corner element array as a substrate suitable for forming retroreflective surfaces, and then removing part of the substrate material comprising the replica to form at least one cavity bounded by side walls in the replica at a depth at least that of the cube corner elements. The replica is then replicated to produce an additional directly machinable substrate suitable for forming retroreflective surfaces, the substrate comprising at least one raised section having side walls at a height at least that of the cube corner elements. Then at least one raised section is directly machined to form a raised zone comprising a plurality of geometric structures including cube corner elements bounded by at least two sets of parallel grooves. The invention comprises a method of manufacturing a cube corner article comprising the steps of providing an initial directly machinable substrate formed as an initial non-unitary cube corner element array comprising a plurality of geometric structures including cube corner elements, and removing part of the substrate material comprising the array to form at least one cavity bounded by side walls in the substrate at a depth at least equal to the height of the cube corner elements. The initial substrate is then replicated to produce an additional directly machinable substrate suitable for forming retroreflective surfaces, with the additional directly machinable substrate comprising at least one raised section having side walls at a height at least that of the cube corner elements. At least one raised section is then directly machined to form a zone comprising a plurality of geometric structures including cube corner elements bounded by at least two sets of parallel grooves. The invention comprises a cube corner article which is a machined replica of a non-unitary initial array comprising geometric structures including cube corner elements. The article has at least one directly machined raised zone of geometric structures including cube corner elements. Brief Description of Drawing Figure 1 is a plan view of a conventional pin bundled full cube corner element array master for manufacturing retroreflective sheeting. Figure 2 is a section view taken along line 2-2 of Figure 1. Figure 3 is a plan view of a conventional pin bundled directly machined cube corner element array master for manufacturing retroreflective sheeting. Figure 4 is a section view taken along lines 4-4 of Figure 3. Figure 5 is a plan view of a retroreflective replica of the master shown in Figure 3. Figure 6 is a section view taken along lines 6-6 of Figure 5. Figure 7 is a plan view of a directly machinable substrate comprising a cavity portion formed in parallel alignment with one of the groove sets formed in the substrate. Figure 8 is a section view taken along lines 8-8 of Figure 7. Figure 9 is a plan view of an additional directly machinable substrate formed by replicating the substrate shown in Figure 7. Figure 10 is a section view taken along lines 10-10 of Figure 9. Figure 11 is a plan view of a directly machined substrate in which a plurality of zones of geometric structures included cube corner elements is shown. Figure 12 is a section view taken along lines 12-12 of Figure 11. Figure 13 is a plan view of a directly machined substrate comprising a plurality of zones of geometric structures and intersecting raised zones. Figure 14 is a section view taken along lines 14-14 of Figure 13. Figure 15 is a plan view of a directly machined cube corner article which is a replica of a zoned substrate formed by directly machining a series of substrates. Figure 16 is a section view taken along lines 16-16 of Figure 15. Figure 17 is a plan view of a directly machined cube corner article comprising a plurality of zones of retroreflective cube corner elements having diverse cube geometry and orientation, including one raised zone. Figure 18 is a section view taken along lines 18-18 of Figure 17. Figure 19 is a plan view of a directly machined cube corner article comprising a plurality of zones of retroreflective elements having different geometries, including at least one raised zone. Figure 20 is a section view taken along lines 20-20 of Figure 19. Figure 21 is a plan view of a directly machined cube corner article comprising a plurality of zones of geometric structures including retroreflective cube corner elements, including one zone comprising cube corner elements having heights greater than cube corner elements in adjacent zones. Figure 22 is a section view taken along lines 22-22 of Figure 21. Figure 23 is a plan view of a directly machined cube corner article comprising a plurality of retroreflective cube corner elements and one raised section. Figure 24 is a section view taken along lines 24-24 of Figure 23. Figure 25 is a plan view of a directly machined cube corner article comprising a plurality of zones of geometric structures including retroreflective cube corner elements, and one raised zone not bounded by grooves in a groove set. Figure 26 is a section view taken along lines 26-26 of Figure 25. Figure 27 is a plan view of a directly machined cube corner article comprising a plurality of zones of geometric structures including retroreflective cube corner elements, and a plurality of multiple non- interfering raised zones. Figure 28 is a section view taken along lines 28-28 of Figure 27. Figure 29 is a section view taken along lines 29-29 of Figure 27. Figure 30 is a plan view of an initial pin bundled directly machinable substrate in which a plurality of geometric structures have been formed by directly machining one set of parallel grooves in the substrate. Figure 31 is a section view taken along lines 31-31 of Figure 30. Figure 32 is a section view taken along lines 32-32 of Figure 30. Figure 33 is a plan view of a replica of the substrate of Figure 30. Figure 34 is a section view taken along lines 34-34 of Figure 33. Figure 35 is a section view taken along lines 35-35 of Figure 33. Figure 36 is a plan view of the replica article shown in Figure 33, comprising additional grooves formed in raised sections within the orientation of the initial groove set. Figure 37 is a section view taken along lines 37-37 of Figure 36. Figure 38 is a section view taken along lines 38-38 of Figure 36. Figure 39 is a plan view of a directly machined two groove set cube corner article comprising a plurality of zones of retroreflective cube corner elements. Figure 40 is a section view taken along lines 40-40 of Figure 39. Figure 41 is a section view taken along lines 41-41 of Figure 39. Figure 42 is a plan view of a pin bundled full cube array with a directly machined cavity. Figure 43 is a section view taken along line 43-43 of Figure 42. Figure 44 is a plan view of a cube corner article formed as a machined replica of the array shown in Figure 42. Figure 45 is a section view taken along line 45-45 of Figure 44. Figure 46 is a section view of a cube corner article having a plurality of zones of geometric structures including raised zones and cube corner elements which form boundary edges of separation surfaces. Figure 47 is a section view of a cube corner article comprising a plurality of zones of geometric structures including raised zones suitable for holding a sealing medium above geometric structures in at lease one other zone. Figure 48 is a section view of a cube corner article comprising a plurality of raised zones and including a plurality of raised sections suitable for holding a sealing medium above zones comprising retroreflective surfaces of geometric structures. Detailed Description of Illustrative Embodiments The manufacture of retroreflective cube corner element arrays is accomplished using either unitary or non-unitary, i.e. assembled, molds made by different techniques. These techniques include, inter alia, those known as pin bundling and direct machining. Assembled molds manufactured using pin bundling, such as initial non-unitary master mold 2 shown in Figure 1 and Figure 2, are made by assembling together zones 4 of individual pins which each have an end portion shaped with features of a cube corner retroreflective element, as shown by full cube corner elements 6, 8. Certain pin bundled arrays permit elaborate assembly into various pin structural configurations. U.S. Patent No. 3,926,402 (Heenan et al), is one example of pin bundling. Direct machining is often a preferred method for efficiently manufacturing master molds for small microcube arrays. This is due to the advantages derived from directly machined substrates in the production of thin replica arrays with improved flexibility, and the often relatively more efficient manufacturing steps when compared with pin bundling. An example of a direct machined substrate is taught in U.S. Patent No. 3,712,706 (Stamm). The Stamm patent and U.S. Patent No. 4,588,258 (Hoopman) are each examples of structures formed by single or multiple passes of a machine tool having two opposing cutting surfaces for cutting grooves to form cube corner optical faces in a substrate. It is recognized that directly machined grooves are preferably machined as groove sets comprising a plurality of separate and parallel grooves. In the direct machining patent examples cited above, at least three groove sets are required. However, examples of direct machining involving only two sets of grooves are shown in U.S. Patent No. 4,349,598 (White) and U.S. Patent No. 4,895,428 (Nelson et al). Retroreflective cube corner element arrays are typically derived from matched pairs of cube corner retroreflecting elements, i.e. cubes which are geometrically congruent and rotated 180°, such as cube corner element 12 and cube corner element 14 shown in directly machined pin bundled cube corner article 16 of Figure 3, which is similar to the non- unitary substrate shown in U.S. Patent 4,243,618 (Van Arnam). The cube corner elements in article 16 are bounded by grooves having identical groove depths, and are the same element length. The highest points in conventional three groove arrays are defined by the cube peaks 20. All of the elements in article 16 are the same height above a common reference plane 18, as shown in Figure 4. Other examples of this fundamental matched pair concept relating to conventional cube arrays is shown in U.S. Patent No. 3,712,706 (Stamm), U.S. Patent No. 4,588,258 (Hoopman), U.S. Patent No. 1,591,572 (Stimson), U.S. Patent No. 2,310,790 (Jungerson), and U.S. Patent No. 5,122,902 (Benson), and German patent reference DE 42 42 264 (Gubela). Referring again to Figure 3 and Figure 4, one example of conventional non-canted cube corner elements is shown having three sides when viewed in plan view, and having an equilateral triangle formed at the base of each cube corner reflecting element. These cube corner reflecting elements are formed by three groove sets directly machined into a substrate. Figure 3 shows a plan view of a directly machined cube corner article useful as a non-unitary master mold which is then replicated, or plated, to form a unitary cube corner article 22 as shown in Figure 5 and Figure 6. Referring again to Figure 3, the grooves 25 in non-parallel groove sets mutually intersect at representative locations 27. Figures 3 and 4 disclose cube corner element retroreflective arrays comprising non-canted cubes which have individual symmetry axes 19 that are perpendicular to a plane 18. The symmetry axis is a central or optical axis which is a trisector of the internal or dihedral angles defined by the faces of the element. However, in some practical applications it is advantageous to cant or tilt the symmetry axes of the matched pair of cube corner retroreflective elements to an orientation which is not perpendicular to the base plane. The resulting canted cube-corner elements combine to produce an array which retroreflects over a wide range of entrance angles. This is taught in U.S. Patent No. 4,588,258 (Hoopman), and is later shown below in relation to other figures. Canting may be in either a forward or backward direction. The Hoopman patent includes disclosure of a structure having an amount of cant up to 13° for a refractive index of 1.5. Hoopman also discloses a cube with a cant of 9.736°. This geometry represents the maximum forward cant of cubes in a conventional directly machined array before the grooving tool damages cube optical surfaces. The damage normally occurs during formation of a third groove when the tool removes edge portions of adjacent elements. U.S. Patent No. 2,310,790 (Jungerson) discloses a structure which is canted in a direction opposite that shown in the Hoopman patent. For these conventional arrays, optical performance is conveniently defined by the percent of the surface area that is actually retroreflective, i.e. which comprises an effective area of active aperture. The percent active aperture varies as a function of the amount of canting, refractive index, and the entrance angle. At non-zero entrance angles, conventional directly machined arrays display, at most, two different aperture shapes of roughly similar size. These result from the single type of geometrically congruent matched pairs of conventional cube corner elements. Canted conventional cube corner arrays exhibit similar trends, although the shape of the aperture is affected by the degree of canting. Some conventional cube corner arrays are manufactured with additional optical limitations, perhaps resulting from canting or other design features, to provide very specific performance under certain circumstances One example of this is the structure disclosed in U.S. Patent 4,895,428 (Nelson et al), and which is shown in a multiple zone modified configuration in several figures below. In these geometries, the cube corner elements are each canted in a backward direction to the point that each of the base triangles is eliminated. Referring again to conventional arrays, U.S. Patent Nos. 4,202,600 (Burke et al), and 4,243,618 (Van Arnam) disclose, and incorporate by reference, the triangular based cube corner reflecting elements or prisms shown in Stamm. The Burke et al. patent discloses tiling of these prisms in multiple differently oriented zones to produce an appearance of uniform brightness to the eye when viewed at a high angle of incidence from at least a minimum expected viewing distance. The Van Arnam reference discloses use of pin bundling to create disoriented patterns of cube corner trigonal pyramids and cutting a grid of grooves into a mold formed by the bundled pins. In this manner, the pins may be cut so that sheeting formed from the molds contains raised grids for bonding a backing material to the sheeting. Some pin bundled retroreflective articles also comprise a grid or ridge-like structure, such as the examples shown in U.S. Patents 4,243,618 (Van Arnam), 4,202,600 (Burke et al), 4,726,706 (Attar), 4,208,090 (Heenan), 4,498,733 (Flanagan), 3,922,065 (Schultz), 3,417,959 (Schultz), and 3,924,929 (Holmen). Another ridge-like structure in a retroreflective article is taught, primarily, for a microsphere or beaded sheeting construction, in U.S. Patent No. 4,025,159 to McGrath. Ridge-like structures are utilized in these examples to provide raised grids for bonding a backing material to the sheeting. Another example of ridge-like structures in pin bundled retroreflective articles is shown within U.S. Patent 3,632,695 (Howell), in which each ridge-like structure is shaped as a lens area to transmit, rather than reflect, light from a source. The invention comprises retroreflective cube corner articles and sheetings, and methods of manufacture, which substantially advance the state of the art of cube corner elements. This results from use of novel manufacturing processes, and directly machined cube corner article designs which greatly enhance the retroreflective performance and produce arrays having novel raised zones. Figure 7 is a plan view and Figure 8 is a section view of a replica 70 of a directly machinable substrate having geometric patterns which are similar, in part, to the patterns shown in sheeting 22 shown in Figure 5. In this embodiment, replica 70 comprises zone 73 having a plurality of geometric structures including identical cube corner elements, such as individual elements 75. Part of the substrate material is removed to form at least one cavity 77 bounded by a base 78 and side walls 79 in the substrate, as shown in the section view of Figure 10. Side walls 79 are machined to a depth D' which is at least that of the depth D" of the initial sets of parallel grooves. In addition to using the preferred substrate materials discussed below, it must be possible to separate replicas from the original pattern or substrate. In some cases, this requires the use of a parting layer between the original and the replica substrates. The parting layer permits separation of replicas by preventing adhesion between the materials of the original and replica materials. Parting layers may consist of a variety of materials such as an induced surface oxidation layer, an intermediate thin metallic coating, chemical silvering, or combinations of different materials and coatings. An additional unitary substrate is then formed as a replica 80, as shown in Figure 9, of directly machinable replica 70. Selection of an appropriate additional unitary substrate must take into account the requirements of replication accuracy of features in the initial substrate, the suitability of the additional unitary substrate for formation of geometric structures including retroreflective cube corner elements, and the ability to separate the additional substrate from the initial substrate without damage to any geometric feature. A non-unitary initial substrate, a unitary replica 70, or a unitary replica 80 is each preferably formed of material suitable for creating retroreflective surfaces in this embodiment. A substrate suitable for forming retroreflective surfaces according to this invention may comprise any material suitable for forming directly machined grooves or groove sets. Suitable materials should machine cleanly without burr formation, exhibit low ductility and low graininess, and maintain dimensional accuracy after groove formation. A variety of materials such as machinable plastics or metals may be utilized. Suitable plastics comprise thermoplastic or thermoset materials such as acrylics or other materials. Suitable metals include aluminum, brass, nickel, and copper. Preferred metals include non-ferrous metals. Preferred machining materials should also minimize wear of the cutting tool during formation of the grooves. As a result of cavity 79 being formed in the replica of the initial directly machinable substrate, unitary replica 80 comprises at least one raised section 100 as shown in Figure 9 and Figure 10. Additional grooves and/or cavities may then be directly machined into replica 80, or multi-generational unitary replicas, to form a plurality of zones of geometric structures including cube corner elements 75 bounded by at least two sets of parallel grooves, as discussed below. It is recognized that while the above embodiment uses an initial non-unitary substrate similar to that shown in Figures 3 and 4, the type of cube corner array shown in Figures 1 and 2 may also be used as an initial non-unitary substrate. It is further recognized that the machining techniques described below may utilize any of the various known types of assembled non-unitary substrates including the types described above for initial non-unitary substrates. Non-unitary initial substrates may even comprise geometric structures which are not cube corner elements. Figure 7 discloses replica joining line 76 of a joining line from an initial non- unitary substrate. Replica joining lines may or may not be apparent in replicas of an initial non-unitary substrate according to this invention. Figure 11 and Figure 12 disclose in plan view and section view respectively another embodiment of the invention in which an ac tional directly machinable unitary substrate 140 comprises zones of cube corner elements including zone 142 and zone 146. Zone 146 may be originally formed as a raised section which is then directly machined using a three groove set pattern. The direct machining of a raised section produces a raised zone, which comprises a plurality of geometric structures including cube corner elements bounded by at least two sets of parallel grooves. In one embodiment, such as that shown in Figure 12, the bottom of the deepest groove in at least one raised zone 146 is machined to a depth which is higher relative to a common reference plane 151 than the highest structure in any rone which is adjacent to the raised zone. Figure 13 and Figure 14 disclose in plan and section view respectively a substrate 160 comprising a plurality of intersecting raised zones 146, also manufactured using a three groove set pattern. As shown in Figures 11-14 for embodiments including directly machined raised zone arrays, the groove sets in a raised zone are preferably parallel to at least one groove set in zones, e.g. portions of the array, adjacent to raised zones. Also, the total width of a raised zone is preferably an integral multiple of the distance between grooves in groove sets in zones adjacent to the raised zone. This is achieved, in one embodiment, by creating an initial substrate with a cavity suitable for forming a raised section which is bounded by grooves from at least one groove set in a first cube corner element array zone. This is particularly useful when the cubes in adjacent zones are the same geometry but different size, i.e. geometrically similar. This results in fewer retroreflective elements which are damaged during the manufacturing process and therefore considerably improves the performance of retroreflective sheetings using this construction. In addition, the machining of raised zones does not initially require machining the surface of a substrate with the same high measure of flatness as when manufacturing raised sections. Conventional cube corner retroreflective element designs include structural and optical limitations which are overcome by use of these raised zone cube corner retroreflective element structures and methods of manufacture. Use of this new class of retroreflective cube corner element structures and manufacturing methods permits diverse cube corner element shaping. For example, cubes in a single array may be readily manufactured with raised discontinuous geometric structures having different heights or different shapes. Use of these methods and structures also permits manufacture of cube arrays which have highly tailorable optical performance. For example, at many entrance angles, including at zero entrance angle, raised multiple structure arrays outperform conventional arrays by exhibiting higher percent active apertures, multiple active aperture shapes, or by providing improved divergence profiles, or both. Raised multiple structure manufacturing techniques may also produce enhanced optical performance resulting from closely spaced intermixed cubes with different active aperture shapes and sizes. This presents more uniform appearances of raised multiple structure arrays over a wide range of viewing distances under both day and night observation conditions. These advantages of raised multiple structure cube corner elements and zones enhance the usefulness of articles having these features. Such articles include, for example, traffic control materials, retroreflective vehicle markings, photoelectric sensors, directional reflectors, flexible retroreflective arrays, and reflective garments for human or animal use. As discussed above, many limiting cases of conventional cube corner element design are surpassed through use of raised multiple structure methods of manufacture. In some raised multiple structure designs, such as that shown in substrate 140 in Figure 11, cube surfaces having some conventional cube geometries may occur as part of a plurality of cube types in a single array. However, the normal limits of conventional cube shapes and performances are not similarly bounded using raised multiple structure methods and structures. Figure 15 and Figure 16 are plan and section views respectively of an alternate embodiment substrate 200, which is a machined replica of a two groove set modified unitary substrate formed from an initial non-unitary cube corner element array. Substrate 200 comprises a plurality of zones 206, 208 of geometrically similar cube corner elements 212, 216. Substrate 200 includes a raised zone 208 which comprises a plurality of geometric structures including cube corner elements 212 which are a different size and which are at a different height above a common reference plane 214 than cube corner elements 216 in zone 206. Substrate 200 is particularly useful in applications requiring high brightness at high entrance angles such as pavement markers, approach markers, channel markers, roadway dividers, barriers, and similar uses. Figure 17 and Figure 18 are plan and section views respectively of another alternate embodiment substrate 250, which is a machined replica of a three groove set modified unitary substrate formed from an initial non-unitary cube corner element array. Substrate 250 comprises a plurality of zones 252, 254, including at least one raised zone. Raised zone 254 comprises a plurality of geometric structures including cube corner elements 260 which are a different size and shape, and are at a different height above a common reference plane 263, than cube corner elements 265 in zone 252. Raised zone 254 is shown with one secondary groove set having directly machined secondary grooves 266 in parallel relation with grooves in an adjacent zone. In this embodiment, two of the grooves in the adjacent zone bound raised zone 254 so that the total width of the raised zone is an integral multiple of the distance between the grooves in the groove set in the zone adjacent to raised zone 254. Another secondary groove set having directly machined secondary grooves 267 is arranged in non-parallel relation with any grooves in adjacent zones. Grooves 268 in a primary groove set are also arranged in non-parallel relation with any grooves in adjacent zones. It is recognized that any of the grooves may be designated for parallel alignment with grooves in an adjacent zone, depending on the desired orientation.This permits orientation of cube corner elements 260 in virtually any manner to optimize optical performance, however, this is accomplished without damage to any structures in adjacent zone 252. Figure 17 further discloses a raised zone multiple structure cube array 250 having at least one zone 254 in which primary grooves 268 do not pass through the secondary grooves 266, 267 at the mutual intersection locations 269 of the secondary grooves. Primary grooves 268 are equally spaced and centered on secondary groove intersection locations 269. Array 250 presents yet another novel feature of raised multiple structure cube corner technology. In particular, a method is disclosed for manufacturing a cube corner article by directly machining three non- parallel non-mutually intersecting sets of grooves. Preferably, these sets intersect at included angles less than 90°. It is recognized that certain machining imprecisions may create minor, unintentional separation between grooves at intersections. However, this aspect of the invention involves intentional and substantial separation. For example, a separation distance between the intersections of the grooves within two groove sets with at least one groove in a third groove set which is greater than about 0.01 millimeter would likely provide the advantages of this feature. However, the precise minimum separation distance is dependent on the specific tooling, substrate, process controls, and the desired optical performance sought. Non-mutually intersecting groove sets create multiple different geometric structures including cube corner elements with different active aperture sizes and shapes. Entire arrays, such as array 250, may even be formed with cube corners created by a combination of mutually and non-mutually intersecting groove sets. The position of the groove sets is controlled to produce maximum total light return over a desired range of entrance angles. Also the distance between grooves in at least one groove set might not be equal to the distance between grooves in at least another of the groove sets. It is also possible to machine at least one set of parallel grooves into a substrate in a repeating fashion with the set comprising a distance between grooves which is optionally variable at each machining of the set. Also, a portion of any one of the grooves may be machined to a depth that is different from at least one other groove depth. Figure 19 and Figure 20 are plan and section views respectively of another alternate embodiment substrate 270, which is a modified replica of a mixed two groove set and three groove set modified unitary substrate formed from an initial non-unitary cube corner element array. Substrate 270 comprises a plurality of zones 274, 276, including at least one raised zone. Raised zone 276 comprises a plurality of geometric structures including cube corner elements 280, formed with three groove sets, which are a different size and shape, and are at a different height above a common reference plane 283 than cube corner elements 285, formed with two groove sets, in zone 274. Indeed, raised zone 276 comprises grooves which are machined to a depth which is higher relative to common reference plane 283 than the highest structure, e.g. cubes 285, in the adjacent zone 274. Substrate 270 comprises cube corner elements which are specifically tailored to provide peak light return at high entrance angles, although other combinations are also useful. Figure 21 and Figure 22 are plan and section views respectively of another alternate embodiment substrate 290. Substrate 290 comprises a plurality of zones 293, 295, including at least one raised zone. Raised zone 295 comprises a plurality of identical geometric structures including cube corner elements 297. Cube corner elements 297 and cube corner elements 293 share a common base reference plane 300, which aids considerably in processing the article. Cube corner elements 297 are a different size, and have peaks at a different height above a common reference plane 300, than cube corner elements 303 in zone 293. Substrate 290 comprises certain structures that are higher than others to help minimize damage to cubes during processing and handling of the replica. Figure 21 and Figure 22 show spacing W between grooves in groove sets in zones adjacent to the raised zone, and the corresponding spacing 2W between grooves bounding the raised zone. It is desirable to use this machining method which results in an article having at least one raised zone with directly machined cube corner elements in which the groove sets in a raised zone are parallel to a groove set in at least on portion of the article adjacent to the raised zone, and the distance between grooves in a groove set in a raised zone is an integral multiple of the distance between grooves in groove sets in areas in at least one portion of the article adjacent the raised zone. This manufacturing innovation permits significant reduction and/or elimination of damage to optical structures adjacent to the raised zones. Variable groove spacing within any groove set may also be used to produce raised multiple structure cube arrays with additional beneficial features. In such cases, the spacing of the primary grooves within a groove set relative to the secondary groove intersections is varied in a repeating pattern throughout array. A wide range of aperture sizes and shapes will result in this array, with a corresponding improvement in the uniformity of the return energy pattern or divergence profile of the retroreflected light due to diffraction. Proper placement of grooves can be utilized advantageously during design to provide optimum product performance for a given application. Another beneficial feature includes manufacture of a raised zone having cube corner elements which are of substantially identical shapes to cube corner elements in portions of the array adjacent to the raised zone, but with the raised zone cube corner elements exhibiting different optical performance than cube corner elements in the adjacent portions of the array. Raised sections and raised zones may be manufactured in different shapes using the methods of this invention, as shown in Figure 23 and Figure 24, in which a six sided raised section 315 is formed in substrate 319. Raised section 315 is surrounded by zone 322 having a plurality of cube corner elements 325. Raised section 315 is manufactured by replicating a modified replica of an initial non-unitary substrate. Part of the substrate material in the modified replica is removed to form at least one cavity. The cavity is formed using any known technique, such as electrical-discharge machining, photo-etching, or other precision techniques. The cavity is bounded by .side walls in the replica at a depth at least that of the cube corner elements formed by the groove sets in adjacent areas. The replica is then replicated to produce a cube corner article comprising a zone 322 and at least one raised section 315 having side walls at a height at least that of the height of cube corner elements formed in the adjacent zone. Figure 25 and Figure 26 disclose a raised zone article similar to the raised section article shown in Figure 23 and Figure 24, but with a raised zone shape which is not bounded by a groove in a groove set. Substrate 330 comprises a raised zone 333 having a plurality of geometric structures including cube corner elements 335. The raised zone is surrounded by adjacent zone 338 having a plurality of cube corner elements 340. In the embodiment of Figures 25-26, cube corner elements 335, 340 are geometrically similar. It is recognized, however, that cube corner elements in the zones may have diverse geometries and orientations to control optical performance characteristics and may be positioned at different heights relative to common reference plane 341. The invention permits numerous combinations of structures previously unknown and not possible within the art of retroreflective cube corner element design and manufacturing technologies. Figures 27-29 disclose views of substrate 350 in which there is formed a plurality of both different and repeating patterns of geometric structures including cube corner elements in multiple independent discontinuous raised zones 352, 354. A portion of a zone may be separated from another portion of the zone by other structures such as a raised section or a raised zone. All portions of a zone should be manufactured at the same time and must not interfere with the machining of any other raised structure. This multiple independent zone capability effectively reduces the number of replication cycles necessary to produce arrays having greater than two zones. The raised zones are bordered by an adjacent zone 365 having a plurality of cube corner elements 370. Figures 30-32 disclose an initial pin bundled non-unitary substrate 390, comprising a plurality of pins 400, in which one initial groove set 398 is machined in initial raised areas 395. Substrate 390 may either have initial recessed areas 393 or it may require forming these areas after machining groove set 398. Figures 33-35 disclose views of a replica 402 of machined substrate 390. In replica 402, the features of substrate 390 are inverted so that the grooves formed by groove set 398 are now peaks in a zone 406, which is lower than adjacent zone 410. Figures 36-38 disclose substrate 402 which is further machined with additional grooves 417 to produce a plurality of machined raised sections 415. Machined raised sections 415 each have structures which share a base plane 421 which is higher than the base plane 424 of the similar structures in adjacent zones. Also, the peak height of structures in adjacent zones is the same. This is also shown in Figure 40. Figures 39-41 each disclose substrate 402 which is further machined with an additional groove set comprising a plurality of grooves 427 to form zones of cube corner elements. Zone 436 comprises cube corner elements 438, and zone 442 comprises cube corner elements 445. The method disclosed in Figures 30-41 produces the zoned cube corner articles of Figures 39-41 using only one replication step. Another embodiment of the invention comprises manufacture of a raised zone cube corner article which also requires only one replication step after machining an initial non-unitary substrate. Referring to Figure 42 and Figure 43, initial pin bundled full cube corner element array 447 is shown in plan and section view respectively. Array 447 comprises a plurality of geometric structures including cube corner elements 448. Part of the directly machinable substrate material comprising the initial non-unitary array is removed to form at least one cavity 449 bounded by side walls 450 in the substrate at a depth at least equal to the height H of the cube corner elements. Replication of the initial substrate is then performed to produce an additional directly machinable substrate 452, shown in Figure 44 and Figure 45, which is suitable for forming retroreflective surfaces. The replica comprises at least one raised section having side walls at a height at least that of the cube corner elements. Direct machining using the various techniques described above of at least one raised section then forms a raised zone 454 comprising a plurality of geometric structures including cube corner elements bounded by at least two sets of parallel grooves. In this embodiment, a taper of approximately 5 * is shown for ease of separation of a replica. Figure 46 discloses a section view of a substrate 455, manufactured as described above as a machined replica of a modified unitary substrate formed from an initial non-unitary cube corner element array. Substrate 455 comprises zones of geometric structures including cube corner elements having different heights and different geometries. Figure 46 shows a plurality of geometric structures, such as structures 459, 460, each comprising a lateral face 461, 462 formed by a groove in a groove set. In at least one zone, lateral faces of the geometric structures form boundary edges 463 of a separation surface 466. The lateral faces may include cube corner element optical surfaces as well as non-optical surfaces on cube corner or other geometric structures. A separation surface 466 may have flat or curved portions when viewed in cross section. Other embodiments of this method include creation of an article, or replicas of the article, which further modify the shape of the retroreflected light pattern. These embodiments comprise, for directly machined arrays, at least one groove side angle in at least one set of grooves which differs from the angle necessary to produce an orthogonal intersection with other faces of elements defined by the groove sides. This is also stated in terms of a sheeting having a plurality of either directly machined or pin bundled cube corner elements each having at least one dihedral angle which is not 90 * . Similarly, at least one set of directly machined grooves may comprise a repeating pattern of at least two groove side angles that differ from one another. This feature may also be stated in terms of a sheeting having a plurality of either directly machined or pin bundled cube corner elements each having, in a repeating pattern, at least one dihedral angle which is not 90°. Shapes of grooving tools, or other techniques, may create cube corner elements in which at least a significant portion of at least one cube corner element optical face on at least some of the cubes are arcuate. The arcuate face may be concave or convex. The arcuate face, which was initially formed by one of the grooves in one of the groove sets, is flat in a direction substantially parallel to said groove. The arcuate face may be cylindrical, with the axis of the cylinder parallel to said groove, or may have a varying radius of curvature in a direction perpendicular to said groove. Raised zone multiple structure geometries are particularly beneficial for use in applications requiring retroreflective sheeting having substantial total light return, such as traffic control materials, retroreflective vehicle markings, photo-electric sensors, signs, internally illuminated retroreflective articles, reflective garments, and retroreflective markings. The enhanced optical performance and design flexibility resulting from raised zone multiple structure techniques and concepts relates directly to improved product performance, cost efficiencies, and marketing advantage. Total light return for retroreflective sheeting is derived from the product of percent active aperture and retroreflected light ray intensity. For some combinations of cube geometries, entrance angles, and refractive index, significant reductions in ray intensity may result in relatively poor total light return even though percent active aperture is relatively high. One example is retroreflective cube corner element arrays which rely on total internal reflection of the retroreflected light rays. Ray intensity is substantially reduced if the critical angle for total internal reflection is exceeded on any one of the cube faces. Metallized or other reflective coatings on a portion of an array may be utilized advantageously in such situations. For example, a particular raised zone which has cube surfaces contacting a sealing medium will often be more reflective when the surfaces have a reflective coating. Alternately, a portion may comprise an entire array. Separation surfaces may be advantageously utilized to increase light transmission or transparency in sheeting, including flexible sheeting, utilizing raised structure or multiple zone retroreflective cube corner element arrays. For example, this is particularly useful in internally illuminated retroreflective articles such as signs or automotive signal light reflectors, which are normally manufactured using injection molding. Retroreflective directly machined cube corner articles are often designed to receive a sealing film or backing material which is applied to the retroreflective article in order to maintain a low refractive index material, such as air, next to the retroreflective elements for improved performance. In conventional arrays this medium is often placed in direct contact with the cube corner elements in ways which degrade total light return. However, using raised zone multiple structure constructions, a sealing medium may be placed on the highest surface of the array without contacting and degrading the optical properties of lower retroreflective cube corner elements. The highest surface may comprise cube corner elements, non-retroreflective pyramids, frustums, posts, or other structures. Although slight height variations may result from slight non-uniformity of groove positions or included angle of cube corner elements due to machining tolerances or intentional inducement of non- orthogonality, these variations are not analogous to the variations disclosed and taught in this invention. For arrays using a sealing medium, the highest surfaces may be truncated both to hold the medium above the cube corner elements as well as to increase the light transmissivity of the sheeting. Light transmissivity of the sheeting may be increased through use of a transparent or partially transparent sealing medium. Articles manufactured according to the methods of this invention are useful for minimizing the contact of a sealing medium with retroreflective cube corner elements. Figure 47 discloses one embodiment of a substrate 470 having a plurality of zones of geometric structures including cube corner elements. A first raised zone comprises cubes 473 which have a height above cubes 475 in another zone. The taller geometric structures, such as cubes 473, provide support for a sealing medium 477 spaced above the lower geometric structures. In similar fashion. Figure 48 shows substrate 481 which, in addition to the geometric structures shown in Figure 47, also comprises raised sections 484. Raised sections 484 are suitable for supporting sealing medium 477 above all other geometric structures including cube corner elements 473, 475. Raised sections 484 may also be advantageously utilized to increase light transmission or transparency in sheeting. Organic or inorganic transparent materials are suitable materials for retroreflective articles or sheeting of this invention. Preferable organic materials include polymers, including thermoset and alkyd materials, thermoplastic materials, and certain mixtures of polymers. Preferably, transparent materials which are dimensionally stable, durable, weatherable, and easily replicated into the desired configuration are used. Illustrative examples of suitable materials include glass; acrylics, which have an index of refraction of about 1.5, such as Plexiglas brand resin manufactured by Rohm and Haas Company; polycarbonates, which have an index of refraction of about 1.59; reactive materials such as taught in United States Patent Nos. 4,576,850, 4,582,885, and 4,668,558; polyethylene based ionomers, such as those marketed under the brand name of SURLYN by E. I. Dupont de Nemours and Co., Inc.; polyesters, polyurethanes; and cellulose acetate butyrates. Polycarbonates are particularly suitable because of their toughness and relatively higher refractive index, which generally contributes to improved retroreflective performance over a wider range of entrance angles. These materials may also include dyes, colorants, pigments, UV stabilizers, or other additives. Transparency of the materials ensures that the separation or truncated surfaces will transmit light through those portions of the article or sheeting. The incorporation of raised sections and /or separation surfaces does not eliminate the retroreflectivity of the article, but rather it renders the entire article partially transparent. In some applications requiring partially transparent materials, low indices of refraction of the article will improve the range of light transmitted through the article. In these applications, the increased transmission range of acrylics (refractive index of about 1.5) is desirable. In fully retroreflective articles, materials having high indices of refraction are preferred. In these applications, materials such as polycarbonates, with refractive indices of about 1.59, are used to increase the differences between the indices of the material and air, thus increasing retroreflection. Polycarbonates are also generally preferred for their temperature stability and impact resistance. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

Claims: What Is Claimed: 1. A method of manufacturing a cube corner article comprising the steps of: a) providing an initial non-unitary cube corner element array comprising a plurality of geometric structures including cube corner elements; b) producing a replica of the cube corner element array as a substrate suitable for forming retroreflective surfaces; c) removing part of the substrate material comprising the replica to form at least one cavity bounded by side walls in the replica at a depth at least that of the cube corner elements; d) replicating the replica to produce an additional directly machinable substrate suitable for forming retroreflective surfaces, the substrate comprising at least one raised section having side walls at a height at least that of the cube corner elements; and e) directly machining at least one raised section to form a raised zone comprising a plurality of geometric structures including cube corner elements bounded by at least two sets of parallel grooves. 2. A method of manufacturing a cube corner article comprising the steps of: a) providing an initial directly machinable substrate formed as an initial non-unitary cube corner element array comprising a plurality of geometric structures including cube corner elements; b) removing part of the substrate material comprising the array to form at least one cavity bounded by side walls in the substrate at a depth at least equal to the height of the cube corner elements; c) replicating the initial substrate to produce an additional directly machinable substrate suitable for forming retroreflective surfaces, the substrate comprising at least one raised section having side walls at a height at least that of the cube corner elements; and d) directly machining at least one raised section to form a raised zone comprising a plurality of geometric structures including cube corner elements bounded b least two sets of parallel grooves. 3. The method of claim 1 in which at least one of the replicas is a unitary substrate. 4. The method of claim 1 in which the geometric structures including cube corner elements in the initial non-unitary array are formed by pin bundling. 5. The method of claim 1 in which the geometric structures including cube corner elements in the initial non-unitary array are formed by direct machining a plurality of groove sets. 6. The method of claim 1 in which the geometric structures in a raised zone include different geometric structures. 7. The method of claim 5 in which the geometric structures in the initial non-unitary array include different geometric structures. 8. The method of claim 1 in which the bottom of the deepest groove in at least one raised zone is machined to a depth which is higher relative to a common reference plane than the highest structure in any portion of the substrate which is adjacent to the raised zone. 9. The method of claim 1 in which the geometric structures in at least one raised zone are formed using at least three groove sets. 10. The method of claim 9 in which the separation between the intersections of the grooves within two groove sets in at least one raised zone and at least one groove in a third groove set is greater than about 0.01 millimeters. 11. The method of claim 9 in which the intersections of the grooves within two groove sets in at least one raised zone are both coincident and non-coincident with grooves in a third groove set. 12. The method of claim 1 further comprising at least one additional machining and replication cycle. 13. The method of claim 5 in which the groove sets in a machined raised zone are parallel to a groove set in at least one portion of the substrate adjacent to the raised zone, and the distance between grooves in a groove set in the raised zone is an integral multiple of the distance between grooves in a groove set in at least one portion of the substrate adjacent to the raised zone. 14. The method of claim 5 in which the cavity is bounded by grooves from at least one groove set in a portion of the initial non-unitary array. 15. The method of claim 5 in which at least one of the cavities is not bounded by at least one groove in at least one groove set of the initial non-unitary array. 16. The method of claim 5 in which at least one of the cavities is formed in the shape of a channel which is aligned with one of the groove sets in a portion of the initial non-unitary array. 17. A cube corner article manufactured according to the method of claim 1 or 2. 18. A cube corner article which is a replica of the article of claim 17. 19. The article of claim 18 which is a retroreflective cube corner sheeting. 20. The sheeting of claim 19 which comprises a structure selected from the group of structures including traffic control materials, vehicle markings, photoelectric sensors, internally illuminated articles, partially light transmissive signs, directional reflectors, garments, and markings. 21. A cube corner article which is a machined replica of a non- unitary initial array comprising geometric structures including cube corner elements, the article having at least one directly machined raised zone of geometric structures including cube corner elements. 22. The article of claim 21 in which the article is a unitary substrate. 23. The article of claim 21 in which the geometric structures including cube corner elements in the initial non-unitary array are formed by pin bundling. 24. The article of claim 21 in which the geometric structures including cube corner elements in the initial non-unitary array are formed by direct machining a plurality of groove sets. 25. The article of claim 24 in which the geometric structures in the initial non-unitary array include different geometric structures. 26. The article of claim 17 further comprising at least one raised section bounded by side walls extending to a height which is higher than any cube corner element in a portion of the article adjacent to the raised section. 27. An article which is a replica of the article of claim 21. 28. The article of claim 27 which is a retroreflective cube corner sheeting. 29. The sheeting of claim 28 in which the sheeting comprises a portion which is reflectively coated. 30. The sheeting of claim 28 in which the sheeting comprises a portion which is transparent. 31. The sheeting of claim 28 which exhibits at least two different active aperture shapes at zero entrance angle. 32. The sheeting of claim 28 in which at least one of the grooves in at least one groove set in at least one zone defines lateral faces of geometric structures which form boundary edges of a separation surface. 33. The sheeting of claim 28 which comprises a structure selected from the group of structures including traffic control materials, vehicle markings, photoelectric sensors, internally illuminated articles, partially light transmissive signs, directional reflectors, garments, and markings. 34. The sheeting of claim 28 comprising geometric structures having multiple heights and in which a plurality of geometric structures provide support for a sealing medium spaced above lower geometric structures. 35. The article of claim 24 in which the groove sets in a raised zone are parallel to a groove set in at least one portion of the article adjacent to the raised zone, and the distance between grooves in a groove set in the raised zone is an integral multiple of the distance between grooves in a groove set in at least one portion of the article adjacent to the raised zone. 36. The article of claim 24 in which the cavity is bounded by grooves from at least one groove set in a portion of the initial non-unitary array. 37. The article of claim 21 in which the geometric structures in a raised zone include different geometric structures. 38. The sheeting of claim 28 comprising at least two adjacent zones, one of which is a raised zone, each comprising a plurality of cube corner elements having substantially identical shapes, but with the adjacent zones exhibiting different optical performance. 39. The sheeting of claim 28 in which the sheeting comprises a plurali r of cube corner elements each having at least one dihedral angle which is not 90°. 40. The sheeting of claim 28 comprising a plurality of cube corner elements each having, in a repeating pattern, at least one dihedral angle which is not 90 * 41. The sheeting of claim 28 in which at least one cube corner element comprises a cube face which is arcuate over a significant portion of the cube face to modify the shape of the retroreflected light pattern. 42. The sheeting of claim 41 in which the shape of the arcuate face is concave. 43. The sheeting of claim 41 in which the shape of the arcuate face is convex. 44. The sheeting of claim 41 in which the shape of the arcuate face is substantially cylindrical. 45. The article of claim 21 further comprising at least one raised section bounded by side walls extending to a height which is higher than any cube corner element in a portion of the article adjacent to the raised section. 46. The method of claim 1 in which the replica step comprises use of a parting layer of material between surfaces being replicated and surfaces of an additional directly machinable substrate.

Inventors: BENSON GERALD M, SMITH KENNETH L, BENSON, GERALD, M., SMITH, KENNETH, L.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1994
Main classification: G02B5
IPC codes: B32B3, G02B5